This invention relates to a color cathode ray tube and, more particularly, it relates to an in-line type color cathode ray tube comprising an in-line type electron gun assembly and showing an improved convergence characteristic.
Generally, an in-line type color cathode ray tube comprises an envelope including a panel 1 and a funnel 2 connected to the panel 1 as shown in FIGS. 1 and 2. A fluorescent screen 3 is arranged on the inner surface of the panel 1, said fluorescent screen 3 having three layers of red (R), green (G) and blue (B) fluorescent materials laid on the inner surface of the panel 1. Additionally, a shadow mask 4 is arranged vis-a-vis the fluorescent screen 3 in close vicinity.
An in-line type electron gun assembly is arranged in the neck 5 of the funnel 2 of the tube and adapted to emit in-line three electron beams.
Additionally, a deflector 6 is arranged around the tube to partly cover the funnel 2 and the neck 5 and a dipole magnet 7 having an N-pole and an S-pole is disposed behind the deflector 6. The dipole magnet 7 is used to regulate the landing beams.
A convergence magnet 8 is arranged outside the neck 5 and comprises at least a pair of ring-shaped magnet plates 11 for generating a quadrupole static magnetic field with two pairs of N- and an S-poles and another pair of ring-shaped magnet plates 10 for generating a hexapole static magnetic field with three pairs of N- and S-poles.
Thus, when the deflector is at rest, the dipole magnet 7 and the convergence magnet 8 converge the three electron beams of a green electron beam operating as center beam and red and blue electron beams operating as side beams that are emitted from the electron gun in array to the center of the fluorescent screen 3 to achieve a satisfactory level of color purity and convergence.
The three electron beams are then deflected by the deflector 6 to scan the fluorescent screen to reproduce the transmitted color image on the fluorescent screen 3.
Since the cathode of the electron gun of an in-line type color cathode ray tube of the above described type is made of a magnetic material, it is apt to be affected by various external magnetic fields including the geomagnetism. Additionally, it is subjected to a different set of external magnetic conditions if it is angularly displaced from the regulated state or used in an geographical area having geomagnetic conditions that are different from those of the area for which it is designed. If, for example, an external magnetic field such as the geomagnetism enters the neck with a component transversal relative to the axis of the color cathode ray tube, the side beams of the three electron beams are subjected to respective forces that are oppositely directed relative to each other. In other words, the side beams are subjected to respective vertical forces, one of which is positively directed relative to the Y-axis while the other is negatively directed relative to the Y-axis so that consequently the red image and the blue image displayed on the fluorescent screen by the side beams can be displaced vertically relative to each other. Thus, a pair of elongated magnets 9 are typically arranged outside the neck oppositely relative to each other on the horizontal axis of the neck and extending along the axis of the tube in order to shield the electron beams against the external magnetic field.
As shown in FIG. 2, the magnets 9 are held in contact with the inner surface of a hollow cylindrical holder H of the convergence magnet 8 and rigidly fitted thereto along the axis of the tube in order to keep them close to the loci of the electron beams in the tube as much as possible.
On the other hand, the hexapole magnet plate 10 generates a magnetic field having a distribution pattern as shown in FIG. 3 by the six N- and S-poles arranged alternately at regular intervals on the ring-shaped magnet plate. Due to the distribution pattern, the magnetic field applies forces to the outer electron beams, or side beams, respectively along a same direction to change the tracks of the side beams. On the other hand, all the forces caused by the magnetic field are set off at central axis of the color cathode ray tube, which agrees with the locus of the center beam, so that the latter is not subjected to any force that can change its course.
As described above, as a convergence magnet for producing a static magnetic field for correcting the loci of the three electron beams and magnets for shutting off external magnetic fields are arranged within the limited area of neck of the color cathode ray tube, the magnets and the magnet plates partly overlap each other along the axis of the tube.
Then, as the magnets and the magnet plates are located close to each other, the magnets can be magnetized further by magnetic poles of the magnet plates, particularly, by those of the hexapole magnet plates.
FIG. 4A of the accompanying drawing shows the distribution pattern of the magnetic field that can be produced by the hexapole magnet plate to correct the electron beams upwardly relative to the vertical axis or in the positive direction of the Y-axis. Note that the positive and negative directions as used herein refers to the direction of the arrow and the opposite direction respectively for both the Y-axis and the X-axis in FIG. 4A.
Referring to FIG. 4A, the N- and S-poles of the hexapole magnet plate 10 are arranged symmetrically on the horizontal axis, or X-axis. Then, the magnets 9A and 9B oppositely disposed on the X-axis are located close to one f the N-poles and one of the S-poles respectively. Thus, as shown in the enlarged partial view in FIG. 4B, the areas of the magnets 9A and 9B located close to the corresponding poles of the hexapole magnet plate 10 respectively will be magnetizes oppositely relative to the polarity of the respective poles of the hexapole magnet plate. Meanwhile, the entire magnets are magnetized along the longitudinal direction, or along the Z-axis, so that consequently each of the magnets give rise to a dipole magnetic field both at the front end, or the end close to the magnet plate, and the rear end. More specifically, an S-pole appears on the surface of the magnet 9A located close to an N-pole of the magnet plate and an N-pole appears on both the front end and the rear end of the magnet 9A, which is arranged on the positive side of the X-axis. Likewise, an N-pole appears on the surface of the magnet 9B located close to an S-pole of the magnet plate and an S-pole appears on both the front end and the rear end of the magnet 9B, which is arranged on the negative side of the X-axis.
Thus, a magnetic field directed from the magnet 9A toward the magnet 9B or from the positive side toward the negative side of the X-axis is generated at the rear end of the magnet 9A and that of the magnet 9B. The generated magnetic field then exerts an upward force on the electron beams passing by the rear ends of the magnets.
Additionally, as the magnetic flux of each of the poles of the magnet plate 10 located on the X-axis is guided by the magnets, the magnetic field generated by the magnet plate 10 and directed from the positive side toward the negative side on the X-axis will be damped. As described above, while the magnet plate 10 is so designed that the magnetic field intensity is reduced to zero on the track of the center beam due to the equilibrated intensities of the magnetic fields of the two poles arranged on the horizontal axis and the four poles located close to the Y-axis without using the magnets, the intensity of the magnetic field generated by the four poles of the magnet plate 10 and directed from the positive side toward the negative side of the X-axis becomes relatively strong when the magnets are arranged because of the damped intensity of the magnetic field on the X-axis. In other words, while a magnetic field is generated and directed from the positive side toward the negative side at the front end as well as at the rear end of each of the magnets, a magnetic field directed from the negative side toward the positive side of the X-axis exists as a total effect of the magnetic fields on the track of the center beam because of the magnetic field generated by the four poles near the Y-axis and directed from the negative side toward the positive side of the X-axis.
Therefore, a magnetic field that is directed from the positive side toward the negative side of the X-axis is generated near the magnet plate 10 on the tracks of the side beams whereas a magnetic field that is directed from the negative side toward the positive side of the X-axis is generated on the track of the center beam. Thus, the two magnetic fields are directed oppositely on the tracks of the side beams and the center beam.
Then, as for the effect of the magnetic fields on the electron beams as observed on the surface of the magnet plate, the side beams are subjected to an upward electromagnetic force, whereas the center beam is subjected to a downward electromagnetic force.
As a result, if the tracks of the electron beams are regulated for a hexapole magnet plate adapted to displace the two side beams by 1.3 mm toward the positive side of the Y-axis in such a way that the center beam is not displaced when no magnets are arranged, then, once the magnets are arranged, the two side beams will be displaced toward the positive side of the Y-axis by 0.5 mm while the center beam will be displaced toward the negative side of the Y-axis by 0.8 mm.
Thus, not only the operability of the magnet plate will be adversely affected by the dipole magnets but also a displacement of the center beam will occur if the six poles are corrected after the operation of regulating the landing beams by means of the dipole magnets so that the regulating operation will have to be repeated to reduce the efficiency of the overall regulating operation.
As discussed above, a known color cathode ray tube is accompanied by a problem that the two side beams show a reduced displacement and the center beam is displaced oppositely relative the side beams in the operation of vertically correcting the tracks of the electron beams for the arrangement of magnets.